Electroforming is a method of making a metal casting of any shape by electroplating rather than traditional machinery. The electroformed process can achieve precision reproduction of complicated shapes and textures, e.g., leather or wood graining. It is known to cast thermoplastic material such as polyvinyl chloride on the inner surface of a thin metal mold member heated to the fusion temperature of the deposited thermoplastic material to form a gel coat which is cured to form a relatively thin plastic shell suitable for use as an outer skin or shell covering on interior products for vehicles, such as door panels, and dash board panels, and on exterior products for vehicles, such as moldings or bumper fascias.
One known method of heating the metal mold member was to direct hot gas over the exterior surface of the heated mold to transfer heat onto the metal of the mold for transfer therethrough into the thermoplastic material (sometimes referred to a plastisol material) to cause the plastisol material, which might be either wet or dry, to gel and cure. In the gel stage, the plastisol material is fused together into a thin layer of material. Thereafter, the fused material is cured at an elevated temperature to form a finished products with desired physical properties.
U.S. Pat. No. 5,032,076 notes that there were attempts to improve the heat transfer rate for directing heat from the heat source to the deposited plastisol material by providing fins on the outer surface of the mold and flowing heated air across the fins, however, past practices of welding or brazing the fins to the mold caused distortion or thermal cracks in the mold during fabrication. An additional problem with known practices of securing fins to the mold was degradation of the properties of the metal in the vicinity of the connection, which could cause grain growth to the metal mold that made it more susceptible to cracking.
A solution to the problem of connecting fins to the mold proposed in U.S. Pat. No. 5,032,076 was to apply a plurality of heat transfer pins, each having an enlarged head juxtaposed against the outer surface of the thin metal mold and connected thereto by a capacitive weld joint.
The present invention provides a solution to the problem of connecting elongated heat transfer fins to the outer surface of a metal mold, without causing any of the problems of mold distortion or thermal cracks noted above.
An object of this invention is to provide an improved mold assembly for molding thermoplastic material having a plurality of elongated heat transfer fins affixed to the outer surface of the shell mold for transferring heat from a heat source to predetermined areas of the inner surface of the shell mold, said fins being connected to the outer surface, without causing distortion or cracking of the mold.
Another object of the present invention is to provide an improved method of fabricating a mold assembly for molding thermoplastic material by forming a plurality of fins each comprising an elongated metal sheet having a discontinuous flange and connecting the discontinuous flange to the shell mold by TIG welding at spaced points.
Other objects and advantages of the present invention will be made more apparent hereafter.